Methods and devices for operating a drive unit of a vehicle are known, in which an output variable of the drive unit, for instance, an engine speed or a torque is predefined. In vehicles having an internal combustion engine, for example, the air supply to the internal combustion engine is set by an actuator. A throttle valve is usually used as the actuator to do this.
For the purpose of economizing on usage, in modem Otto engines in overrun condition, the injection of fuel is shut off, if possible. In this context, the throttle valve is closed. Consequently, the result is a great braking torque of the internal combustion engine, the so-called engine brake. Because of the large engine braking torque, in overrun condition the vehicle clearly loses speed. In certain driving situations this may be undesirable, and the driver must get out of the overrun condition again in a short period of time. If the throttle valve were open in this case, a clearly lower engine braking torque would come about, and the vehicle could be operated in overrun condition longer.
At a high engine speed, if the driver releases the accelerator, the engine normally goes into so-called overrun fuel cutoff, that is, the overrun condition in which no fuel is injected any more. However, as soon as the engine speed drops below a predefined threshold value, or if, for example, catalyst protective functions prohibit the overrun fuel cutoff, fuel injection again takes place, and therewith torque generation again takes place. In this case this is called a firing overrun condition. This is done for various reasons:
1. If the engine speed continues to drop rapidly, then, in the case of maintaining the overrun fuel cutoff, the danger exists that the engine will shut down, since then, in the case of falling below the predefined idling speed, torque cannot be built up fast enough.
2. In idling operation, the engine has to produce just so much torque as is necessary for the compensation of the torque losses that come about, for instance, because of friction or the operation of ancillary components. If the engine speed is only slightly above the predefined idling speed, the torque cannot be abruptly driven down to zero, since otherwise the idling control circuit would become unstable.
3. If the driver releases the accelerator, this may mean that he wishes to stop as quickly as possible. In this case it would be desirable that, while observing the boundary conditions with respect to engine technology and control technology, the lowest possible torque is set. But it may also mean that the driver wishes to roll along as long as possible, using the momentum the vehicle still has. In this case it would be desirable that the burnt fuel is converted to kinetic energy as optimally efficiently as possible. It may even be desirable to combust an increased air quantity and fuel quantity.
Using the vehicle application data, it is uniformly established for all cases what charge and what ignition angle efficiency is to be set, if the firing overrun condition exists. In various driving situations, this is not optimal.
It may happen that the driver wishes to stop because of a red traffic light. In this case, based on an application designed for good rolling behavior, torque is built up unnecessarily which is immediately compensated for by increased braking. This leads to unnecessary fuel usage, and besides that, to increased break wear. Or, the driver might want to use the momentum of the vehicle for rolling, as long as possible. However, if the firing overrun condition is designed for minimal torque, the ignition angle is drawn to the latest possible value, that is, the mixture is combusted at poor efficiency. The driver has to compensate for the deceleration connected with this by stepping on the gas again earlier, which also acts to increase usage.